Many thanks, top tips those.
Used a nice 14mm 6 sided socket and 22mm spanner. Came undone just fine. The stanchion dropped enough to put the new rubber top cover on.
Pushed leg back in, tightened nut to 45Nm.
In case someone else does this, don't forget to put the pry slot in the plastic top cap in a position where you can get to it easily incase it needs removing again in the future.
As it was just this I needed to change I didn't touch the little Allen bleed screw.
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